HPLC vs. LC-MS: How Peptide Purity and Identity Are Tested

Most peptide Certificates of Analysis rely on two techniques: high-performance liquid chromatography (HPLC) and liquid chromatography-mass spectrometry (LC-MS). They answer different questions. HPLC asks "how much of this sample is one compound?" LC-MS asks "what is that compound?" A report that includes only one of them leaves a gap.

What HPLC measures

In HPLC, a dissolved sample is pushed under high pressure through a column packed with a stationary material. Different molecules interact with that material differently, so they leave the column at different times. A detector, usually ultraviolet (UV) at a wavelength where peptide bonds absorb, records each molecule as a peak on a chromatogram.

Purity is reported as the area of the main peak as a percentage of the total area of all detected peaks. A result of 99% means the main peak makes up 99% of what the detector saw.

What HPLC can't tell you

• Identity. A tall, clean peak proves the sample is mostly one compound, not that it's the right compound.
• Things the detector can't see. Water, salts such as counter-ions, and some other impurities don't show up under UV detection, so they aren't counted in the percentage.
• Amount per vial. Purity is a ratio. It says nothing about how many milligrams are in the vial.

What LC-MS measures

LC-MS pairs the separation step with a mass spectrometer. As compounds leave the column, they are ionized and sorted by their mass-to-charge ratio. Peptides often carry several charges at once, so the spectrum shows a series of related peaks that can be converted back to the molecule's mass.

The lab compares this measured mass with the theoretical molecular weight calculated from the peptide's sequence. A close match confirms the compound's identity. LC-MS can also help identify impurities, such as truncated sequences or oxidized forms, that appear as minor peaks in the HPLC trace.

Why a good COA includes both

Used together, the two techniques confirm that the main compound is the expected peptide (LC-MS) and that the sample is mostly that compound (HPLC). For blends, LC-MS should identify every component, since an HPLC trace alone can't show whether each peptide in a mixture is present.

Other tests you may see

• Net peptide content. Measures how much of the powder's weight is actual peptide, as opposed to water and counter-ions.
• Quantity or fill testing. Measures the total amount in the vial against the label.
• Endotoxin testing. Checks for bacterial endotoxins, often with an LAL assay.
• Sterility and heavy metals. Less common for research-grade lyophilized peptides, but sometimes included.

Reading the numbers

When you review a report, check that the batch number matches your vial and the testing lab is named. Then confirm the measured mass lines up with the expected molecular weight, and look at the HPLC purity. Our guide to reading a peptide COA walks through a full report section by section.

Azyrion Labs publishes batch reports from independent laboratories. Our Freedom Diagnostics reports include both HPLC purity and LC-MS identity results. You can find any documented batch in our COA lookup.

All Azyrion Labs products are for laboratory research use only. Not for human or veterinary use.